The invention relates to a method of manufacturing electronic components comprising at least one stripline.
Striplines in a plurality of designs are the most frequently used line types for the circuit technology in the VHF range and the microwave range, as well as in fast digital circuits, in so far as special requirements regarding very low damping levels or a high power transfer do not require the use of coaxial lines or waveguides. The stripline design also enables passive components, such as coils or couplers, to be readily realized.
Couplers are used, inter alia, as high-frequency components for mobile telephones or base stations, enabling decoupling of HF signals between the output of a power amplifier and an antenna. The decoupled signal is used to control the output power of the amplifier. Such a coupler comprises, for example, two coupler loops; one of said loops being the main loop for transmitting the transmit signal with as little loss as possible. The second loop, which is commonly referred to as auxiliary loop, decouples a signal which is small relative to the transmit signal.
Different embodiments of such couplers are known. On the one hand, there are ceramic multilayer couplers. In these ceramic couplers, the electrode structures are printed onto ceramic foils, which foils are stacked and sintered into components. The drawback of this printing process resides in the coarse-grain morphology of the electrodes, which leads to a higher electric resistance. A further drawback is the small accuracy of the printing process. This may lead to scattering of the electronic properties.
On the other hand, there are couplers manufactured in microstrip technology. In 1991 IEEE MTT-S International Microwave Symposium Digest, Vol. II, 856-860, a thin-film coupler is described comprising two striplines serving as coupler loops. Both coupler loops are provided on a dielectric substrate having a high dielectric constant K. On the rear side of the ceramic substrate, there is also a metallic layer which serves as a ground plane. In addition, six end contacts are secured to the component, two of which contact a coupler loop and two contact the ground plane.
It is an object of the invention to provide a method by means of which electronic components comprising at least one stripline, particularly couplers, can be readily and very accurately manufactured at low cost.
This object is achieved by a method of manufacturing electronic components comprising at least one stripline, wherein a metallic base layer is deposited on a substrate layer, a layer of a photoresist is applied to this metallic base layer and provided with a structure, an electroconductive layer is deposited on the exposed areas of the metallic base layer, the photoresist is removed, and the metallic base layer is etched.
This method has the advantage that less metal is required than in other thin-film processes. In the known thin-film processes, first the electroconductive layer is deposited and, subsequently, this layer is structured by means of photolithographic processes. In the method in accordance with the invention, the thin, metallic base layer is provided with an inexpensive photoresist layer, which is subsequently structured. Next, the electroconductive layer is deposited, in the holes of the photoresist, on the exposed, metallic base layer. This method enables compact components of small dimensions to be manufactured.
Preferably, the material for the substrate layer is a ceramic material, a ceramic material having a planarizing layer of glass, a glass-ceramic material or a glass material.
A substrate layer of these materials can be economically manufactured, and the process costs for these components can be kept low.
It is further preferred that Cr/Cu is used as the material for the metallic base layer.
A metallic base layer is provided to facilitate the subsequent deposition of the electroconductive layer and improve the adhesion of the stripline onto the substrate layer.
It is also preferred that Cu, Ag, Al or Au is used as the material for the electroconductive layer.
These materials all are satisfactorily conducting metals.
It is preferred that the striplines are situated in one plane when a component comprises two or more striplines.
These components can be readily manufactured at low cost because the striplines can be manufactured in the same process step.
In the case of a component comprising two or more striplines, it may be preferred that the striplines overlap each other at least once.
By means of overlapping areas of the striplines, a plurality of variations in the length and the geometry of the striplines can be achieved, which fulfill the requirements to be met by the component.
In this embodiment, it is preferred that overlapping areas of the striplines are realized by means of bridges of a conductive material.
By means of bridges, overlapping areas of the striplines can be formed in a simple manner. In addition, the sections of the striplines extending in one plane may be provided on the substrate layer in the same process step. The completion of the relevant stripline by means of a bridge of a conductive material takes place in a later process step.
It is also preferred that each end of a stripline is in electrical contact with a current supply.
At the current supplies, each component can be electrically connected to further components of a switching circuit. Dependent upon the application or the way in which the component is mounted, an electroplated SMD end contact or a bump end contact or a contact surface can be used for the current supply. The use of SMD end contacts or bump end contacts enables discrete components to be manufactured.
It is also preferred that a protective layer of an inorganic material and/or organic material is provided on the component.
By virtue of said protective layer, the component is protected against mechanical stress and corrosion due to moisture.
It may be advantageous that the lower side of the substrate layer is provided with a ground plane. In this advantageous embodiment, it is preferred that the metallic layer is connected to at least one further current supply.